In the area of immigration, "visa" and "status" are simple yet confusing concepts. Often, many people use the word "visa" even when "status" would be a proper term. How are they different?
A visa is an entry ticket that you must present at the U.S. port of entry in order to enter the U.S. Therefore, you have to obtain a visa prior to entry, by applying at one of the U.S. Embassies/Consulates (part of the Department of State) abroad. There are various types of visas, depending on the purpose of your entry---tour, business, study, employment, permanent residence, etc. But having a visa does not guarantee that you will be admitted into the U.S. CBP (part of the Department of Homeland Security) will inspect you at the port of entry, reviewing your passport, visa, and other supporting documents; if the CBP officer determines that your true intent of entry is consistent with the purpose of the visa you have, then you will be admitted into the U.S. The expiration date on your visa determines how long you can use it for entry purposes; it does not determine how long you can stay in the U.S. after admission.
Once the CBP officer admits you into the U.S., you will be given a status, which you must maintain in order to stay in the U.S. You will receive a stamp on your passport, showing what type of status you are given and how long the status is valid. Most of the time, you will be given the same type of status as your visa, e.g., if you arrive with an F-1 visa, you will be given an F-1 status. Depending on your eligibility, you can apply for extension/change of status while in the U.S., which will be reviewed and decided on by USCIS (part of the Department of Homeland Security). Even if your visa (entry ticket) has expired, you can legally stay in the U.S. as long as you are maintaining your status.
Let's say, you are studying in the U.S. in an F-1 status. You are about to graduate, and you just found a U.S. company that is willing to hire you. How can you obtain an H-1B status so that you can legally work in the U.S.? There are many details involved in the process, but if we just focus on the current topic (visa vs. status), there are two ways: you can go back to your home country, apply for and obtain an H-1B visa at the U.S. Embassy/Consulate, and come back to the U.S. with the new visa; or, you can stay in the U.S. and seek to change your status from F-1 to H-1B based on a proper petition by the U.S. company. In the second option, however, even if you successfully change your status, you will still need to apply for a visa at the U.S. Embassy/Consulate later on if you want to return to the U.S. after traveling abroad. Remember, you have to have a visa in order to enter the U.S. from outside.
There are many variations depending on the details of each case. Please consult our office to find out what needs to be done in order to lawfully enter/stay in the U.S.